fix title for 2020/yang

We convert it to use the canonical yyyy.dir form.

2020/yang/.entry.json

@@ -5,7 +5,7 @@
     "year" : 2020,
     "dir" : "yang",
     "entry_id" : "2020_yang",
-    "title" : "yang.2020",
+    "title" : "2020.yang",
     "abstract" : "PIN-protected program generator",
     "author_set" : [
         {